1. Field of the Invention
This invention is applied to grow a ZnSe thin film on glass and GaAs substrates by using the continuous wave (CW) CO2 laser evaporation with ion-assisted system to produce optical multi-layer films, anti-reflection coating and blue light emitting devices.
To grow ZnSe thin films on substrates and deposited plating films then manufacture blue light emitting devices, there are usually two methods applied in the industry. One is using metal organic chemical vapor deposition (MOCVD) and the other is using Molecular Beam Epitaxy (MBE). The former method has been applied on GaAs(100) substrates, and the latter also has been used to deposit some epitaxy films on GaAs substrates or GaN epitaxy films on sapphire substrates.
At the present time, the most popular thin film epitaxy methods are the above-said MOCVD and MBE. The first method is more dangerous because its materials are organic metals; however, the second also has a weakness. It is expensive and thus difficult to mass-produced.
2. Description of the Prior Art
As for MOCVD, organic metal molecules are led through gas ducts into the chamber, and then are deposited on high temperature substrates. Through thermal decomposition, a film will be formed on the surface of substrate. There are many successful cases on growth of a ZnSe thin film. In Appl. Phys. Lett. (Vol. 33, p.656, 1978), Stutius et al. it is stated that they had successfully grown ZnSe thin film on GaAs substrates. They were using Zn(CH3)2 and H2Se as Zn and Se""s precursors. The high quality ZnSe epitaxy films were grew on 300 to 350xc2x0 C. substrates at a growth rate of 2 xcexcm/hr, under a total system pressure of 0.15 torr, at the 5:1 ratio of Se and Zn in gas. According to X-ray survey results, the ZnSe (400) formed had the narrowest width at its Full-Width Half-Maximum (FWHM) peak, a symbol of the best thin film quality. Analyzed by the Photoluminescence measurement in an ambient temperature, it emitted at the near-band gap of 447 nm without remarkable defects. It appeared that the films on the substrate were a high-quality light emitting epitaxy layer with no remarkable defects on it. In addition to Stutius"" study, Simpson and Williams published their MOCVD techniques in 1990 in Appl. Surface Science (Vol. 46, p.27). They used Zn(CH3)2 and Se(CH3CH2)2 as precursors of Zn and Se to grow ZnSe on GaAs (100) substrates at a temperature of 275 to 350xc2x0 C. In such conditions, high-quality single crystal ZnSe was formed.
Apart from growth of high-quality ZnSe thin films, MOCVD is also applied to the field of GaN epitaxy, as one of the blue light emitting devices. In 1997, Kobaashi et al. published in Jpn. J. Appl. Phys. (Vol. 36, p.2592) that using DMHy and Ga(CH3)3 as precursors of N and Ga could improve the disadvantages of high temperature in traditional methods. The traditional methods of growing GaN was under high temperature conditions, especially temperatures higher than 1,000xc2x0 C. Kobaashi""s methods adopted two approaches. One approach was to grow a GaN buffer layer first at a lower temperature of 530xc2x0, then grow the GaN epitaxy at 770xc2x0 C. The other approach was to grow A1N buffer layer at 700xc2x0 C. and then grow GaN at 850xc2x0 C. The results showed that the best film quality could be achieved by the second approach at 850xc2x0 C., V/III ratio of 60 at total 300 Torr system pressure.
Another high quality epitaxy technique is the Molecular Beam Epitaxy (MBE). In ultra high vacuum conditions, atoms are heated into steam and sprayed on substrates to cause interaction and form the epitaxy films. In 1993, Reichow et al. used this technique to form ZnSe epitaxy on GaAs substrates by letting the Zn atom flow first to prevent formation of Ga2Se3. Reichow et al found that they could grow 1-2 xcexcm ZnSe thin film at 331-350xc2x0 C. By measuring with X-ray Diffraction Spectrometer, the FWHM of ZnSe (400) peak was 200 arcsec, an excellent crystal films quality. In 1995, another method was published by Sou et al. on J. Crystal.Growth (Vol. 147, p.39). With preheating to remove oxide contamination of substrate, Sou et al found that they could grow 0.45 to 1.6 xcexcm of ZnSe thin film at a growth rate of 0.18 to 0.36 xcexcm/h at 300xc2x0 C., letting in Zn atom flow first for one minute and then Se atom flow under a constant pressure ratio of 1:33. Analyzed by high-resolution X-ray Diffraction Spectrometer, the half-height width of ZnSe (400) wave also reached 200 arcsec. Both cases showed that MBE can produce very good crystallization of ZnSe thin film.
The same approach can be applied to grow high-quality GaN epitaxy. On Appl. Phys. Lett., Lurobe et al. stated that high-quality GaN epitaxy films were achieved on sapphire substrates at 800xc2x0 C. at a speed of 0.35 xcexcm, using Ga(C2H5)3 and Nitrogen plasma as the resources of Ga and N (Vol. 73, p.2305, 1998). According to the results of X-ray diffraction spectrometer analysis, this approach could also grow high-quality GaN epitaxy.
A bulk crystal ZnSe producing method, covered in U.S. Pat. No. 4,866,007, is stated as follows:
(1) Put the initial material, bar-type polycrystal ZnSe, into the reaction Chamber;
(2) Fill in mixture of Noble Gas, Nitrogen, and H2Se at 0.1 to 100 Torr; and
(3) Move the polycrystal material through a high temperature passage and let it transform into single crystal ZnSe material at a speed of 5 mm/day.
Another approach covered in U.S. Pat. No. 4,584,053 is to prepare ZnSe single crystal for a large area with features of high quality and low impurity. The points are: plate polycrystal ZnSe by CVD, seal it in capsules, and under high temperature and pressure conditions transform it into single crystal material for producing crystal film substrates.
In U.S. Pat. No. 5,174,854, an approach for crystal growth of group II-VI compound semiconductors is stated. It is to dissolve Se in Zn solvent to the extent of saturation under the conditions of high temperature and 6-9 atm, place substrates under 10 w temperature conditions, and thus grow single crystal ZnSe.
However, U.S. Pat. No. 5,616,178 covers the way to plate p-type ZnSe by MOCVD. The initial material used in this approach is p-type organic substance that should have at least something like di-isopropylamine.
A method for growing p-type ZnSe (for producing a zinc selenide blue light emitting device), covered in U.S. Pat. No. 5,192,419, is called Electrochemical Accumulation. The method includes bathing an anode (with Zinc) and single crystal ZnSe substrates in the Zn and Se ion solution. When electricity flows from zinc anode to n-type ZnSe substrate (cathode), p-type ZnSe will accumulate on n-type ZnSe cathode to form a pn-junction ZnSe semiconductor.
A single crystal growth method, using devices to melt substance under high-pressure conditions, such as in a vertical Boogiemen high temperature oven, is covered in U.S. Pat. No. 5,554,219. In this approach, ZnSe is melted at high temperature and then grows into single crystal material through proper procedures.
In U.S. Pat. No. 5,015,327, a method to grow ZnSe thin films on ZnSe substrates is as follows: (1) heat substrates from 250xc2x0 C. to 450xc2x0 C. at 0.1 to 10 Torr of Hydrogen pressure; and (2) fill in Zn(CH3)2 or Zn(C2H5)2 and H2Se gas at a 1:10 to 100 of Se and Zn Mole ratio. In such conditions, ZnSe thin films can successfully grow on single crystal ZnSe substrates.
The purpose of this invention is to use an ion-assisted CW CO2 laser deposition system which can grow ZnSe thin films on substrates to produce optical multilayer films and anti-reflection films for blue light emitting devices.